The biggest cost driver is not the technology stack — it remains how much is still undecided. Every open question in the requirements is converted into padding somewhere in the quote. A vendor that has no visibility into what happens on the unhappy path will assume the more expensive option. Investing a few days in requirements work often reduces the total far more than any rate negotiation.

Integrations tend to be the second big multiplier. A feature that touches only your own data is predictable; the same functionality connected to a payment provider and a CRM is a different problem. The unknown lives in the counterparty: undocumented APIs, waiting on someone else's team, data that does not match your model. Ask the estimator to list every external system, because this is the usual source of overruns.

Non-functional requirements silently change the budget. An application support services used by a handful of staff is a very different build from the same functionality handling thousands of external customers. Compliance work, uptime targets, performance under load, data retention rules and multi-language support each add real engineering time. Write them down at the start or expect them to arrive later as change requests.

The mix of people behind the number matters a great deal. A day rate reveals very little on its own: one senior developer at a premium rate can be cheaper per delivered feature than two inexperienced developers who need constant review. Also ask who else is billed: delivery management, QA, release engineering and design are legitimate costs, but these should be named rather than hidden inside a blended rate.

The build price is not the full cost of ownership. Budget for cloud costs, third-party licences, logging and react native development agency alerting and an ongoing support budget annually. A reasonable rule of thumb is that software in active use consumes a recurring percentage of the original budget per year simply to stay current. Leaving it out of the budget remains the most frequent planning error.